The invention relates to a method for managing a battery. The invention also relates to a computer program and a battery management system, which are equipped to carry out the method as well as to a motor vehicle comprising a battery and a battery management system of said type.
In electrically driven vehicles, storage batteries based on lithium chemistry are often used as electrical energy stores (EES) because said batteries have the largest energy density available up until now in comparison to nickel or lead based storage batteries. Very high demands are placed on battery systems that are used in electrically driven vehicles with regard to available energy content, discharge power, charging/discharging efficiency, service life and reliability. A service life of 15 years is expected from these storage batteries.
A trouble-free operation of such battery systems requires the safe and reliable function of the cells, modules and the entire battery pack. In order to achieve this, physical variables such as amperages, voltages, temperatures, insulation resistances and pressures are continually controlled. With the aid of the measured values of these variables, management and operating functions can be implemented with which the warranted durability, reliability and safety of the battery system can be maintained. In order to prevent premature ageing of the battery cells, cell voltage limit values and maximum amperages are predefined.
A root mean square current represents a parameter that is very decisive for ageing. This parameter, i.e., more precisely stated, the root thereof, is also denoted as the IRMS-value. In the case of the root mean square current, high amperages are more strongly weighted than low amperages. The IRMS-value therefore takes into account the development of the temperature gradient within the cell as well as the effects of the deposit of the lithium on the electrodes, so-called lithium plating. The root mean square current is also written as:
            (              I        RMS            )        2    =            1              t        ⁢                                  ⁢        2              ⁢                  ∫        1                  t          +                      t            ⁢                                                  ⁢            2                              ⁢                                    I            2                    ⁡                      (            t            )                          ⁢        dt            
Limit values exist for this parameter to which said parameter is adjusted in the prior art. It is, for example, known from the German patent application DE 10 2011 012 818 A1 how to determine the root mean square current and keep the same below a predefined setpoint value, i.e.(IRMS)2<(IRMS_lim)2 
The WIPO patent application WO 2012/091077 discloses a method in which a state of health (SOH) is ascertained from the root mean square current.
Because the limit value for the root mean square current reaches the limit thereof within a short period of time at low temperatures or when starting to ascend a hill, for example already after 10 seconds, an aim of the invention is increase the limit values without having to modify the warranted battery service life, which, for example, is set at 15 years.